Objective: Previous studies of predominantly ecological design have indicated a possible elevation of gastrointestinal cancer risk in population groups exposed to drinking water contaminated with asbestos from natural sources or asbestos-cement containing water pipes. In the present study the possible effect of ingested asbestos fibers on gastrointestinal cancer risk was investigated in an occupational group where a proportion of the employees was exposed to asbestos in their drinking water.
Method: A cohort of 726 lighthouse keepers first employed between 1917 and 1967 were followed up for cancer incidence from 1960 to 2002. The standardized incidence ratio (SIR) was calculated as the number of new cancer cases divided by the expected number based on five-year age and sex specific incidence rates in the general rural population of Norway. A 95% confidence interval (CI) was calculated for all SIR values assuming a Poisson distribution of the cancer cases.
Results: Risk of stomach cancer was elevated in the whole cohort (SIR: 1.6, CI: 1.0-2.3), in the subgroup with definite asbestos exposure (SIR: 2.5, CI: 0.9-5.5), and when the group was followed for 20 years and more after first possible exposure (SIR: 1.7, CI: 1.1-2.7). Less consistent results were found for colon cancer; SIR was 1.5 (CI: 0.9-2.2) overall, 0.8 (CI: 0.1-2.9) among the exposed, and 1.6 (CI: 1.0-2.5) twenty years and more after first possible exposure.
Conclusion: The results support the hypothesis of an association between ingested asbestos and gastrointestinal cancer risk in general and stomach cancer risk specifically.